Electrochemical Properties Investigation Of MWCNTs/S Based Cathode Materials Composited With BiFeO₃

Recently,the demand of people for energy storage systems with superior performance is increasing rapidly.Lithium-sulfur batteries was believed as one of the most promising substitution of lithium ion batteries because of its high theoretical specific capacity(1675 mAh g^-1)and specific energy(2600 Wh kg^-1).In this work,three kinds of BiFeO₃ were synthesized by different methods and were mixed with MWCNTs/S based cathode materials of lithium-sulfur batteries respectively.Testing the electrochemical properties to investigate the effect of BiFeO₃ with different morphology and ingredient on the electrochemical properties of the electrode materials.The main research works of this thesis are summarized as follows:1.Pure BiFeO₃ powder with specific surface area of 4.84 m² g^-1 were synthesized by hydrothermal method reaction at 200°C for 6 h.The size distribution of BiFeO₃powder was in the range of 0.2¹?m and the morphology of powder was irregular with smooth surface.Adding the BiFeO₃ powder with the weight percentage of 0%?3%?5%?7%and 10%respectively into MWCNTs/S composites,which were used as the cathode materials of lithium-sulfur batteries.At the current density of 0.5 C(1.0C=1675 mA g^-1),the capacity retention rate of these electrode material were 21.37%,40.66%,30.50%,30.30%,29.25%and the corresponding coulombic efficiency were99.04%,98.58%,96.06%,97.74%,96.13%after 100 cycles respectively.It was obvious that the composite electrode materials with 3 wt%BiFeO₃ had the best cycling performance.And mixing BiFeO₃ in the MWCNTs/S cathode materials decreased the coulombic efficiency,which was probably due to the large particle size and high resistivity of BiFeO₃.2.BiFeO₃ powder with specific surface area of 6.25 m² g^-1 were synthesized by sol-gel method.The size distribution of BiFeO₃ powder was between 0.2¹?m,and the morphology of powder was irregular with smooth surface.Mixing the BiFeO₃ into the cathode materials with different percentage as mentioned above.The electrochemical properties results showed the similar change regular observed in the electrode materials mixed with BiFeO₃ synthesized by hydrothermal.Comparing the performance of electrode materials with 3 wt%BiFeO₃ synthesized by sol-gel and hydrothermal,the capacity retention rate were 33.73%,37.78%at the current density of 0.5 C after 100 cycles respectively.The electrode materials electrochemical performance was worse when mixing with BiFeO₃ synthesized by sol-gel,which may be caused by its worse crystallinity and impurity.3.0.86(BiTi_0.1Fe_0.8Mg_0.1O₃)-0.14?CaTiO₃??BTFM-CTO?powder with specific surface area of 6.25 m² g^-11 were synthesized by sol-gel method.The size distribution of BTFM-CTO powder was in the range of 2⁵?m.The results showed that the BTFM-CTO particles had roughness surface and was agglomerated seriously.Mixed the BTFM-CTO powder in the cathode materials with different weight percentage as mentioned above.The electrochemical properties results showed the similar change regular observed in the electrode materials mixed with BiFeO₃.Comparing the performance of electrode materials mixed with 3 wt%BTFM-CTO and BiFeO₃synthesized by sol-gel and hydrothermal,the capacity retention rate were 28.34%,32.30%and 37.78%at the current density of 0.5 C after 100 cycles respectively.The electrode materials mixed with BTFM-CTO had the worst cycling performance,which may be caused by the low crystallinity,larger particle size and impurityof BTFM-CTO.